This invention relates to improved compositions of natural pyrethroid insecticidal substances and a method for preparing such compositions. More particularly, this invention relates to a composition including a mixture of naturally occurring pyrethroid substances containing naturally occurring polyacetylenic substances in which the polyacetylenic substances are inactivated or removed.
Pyrethrum is a naturally occurring mixture of insecticidal substances obtained from the flowers of the tropical chrysanthemum, including Chrysanthemum cinerariifolium, Chrysanthemum coccineum, or Chrysanthemum marshallii. There are six active substances in naturally occurring pyrethrum: pyrethrin I and II, jasmolin I and II and cinerin I and II. The substances have the following structure: ##STR1## wherein R' is CH.dbd.CH.sub.2 (pyrethrin), CH.sub.2 CH.sub.3 (jasmolin), or CH.sub.3 (cinerin) and R is CH.sub.3 (I) or CO.sub.2 CH.sub.3 (II). Pyrethrin I is most active for kill and pyrethrin II is most active for knockdown.
Pyrethrum possesses unusually fast contact insecticide action causing paralysis in a few moments and yet has little mammalian toxicity. Pyrethrum, one of the first widely used insecticides, remains commercially important today. Use of pyrethrum grew steadily until World War II when DDT, an inexpensive insecticide, became readily available. Current concern over the environmental acceptability of synthetic insecticides has renewed interest in pyrethrum.
The poor stability of pyrethrum, which breaks down rapidly upon exposure to oxidating compounds and light, may have less long term environmental impact than long acting substances such as DDT. However, its short active time has seriously limited its large scale use in agriculture. In order to maintain insecticide activity, frequent reapplication of pyrethrum is often necessary.
Although purified pyrethrum compositions are quite stable in dilute solution, decomposition increases rapidly with increasing concentration. Concentrated thin films that remain on plant or soil surfaces after application of the insecticides are largely decomposed after one day. It is well known that ultraviolet light causes extensive decomposition, and naturally occurring oxygen is also thought to be damaging. In the past, pyrethum compositions have been stabilized by the addition of traditional stabilizers, ultraviolet light absorbing substances, and anti-oxidants which have improved stability but have not totally eliminated decomposition.
One would expect that naturally occurring polyacetylene compositions, which strongly absorb ultraviolet light, would contribute to the overall stability of pyrethrum. Polyacetylenic compounds are compounds which contain more than one carbon-carbon triple bond. We have found, surprisingly, that naturally occurring polyacetylenes accelerate pyrethrum decomposition. Most chrysanthemums contain polyacetylenes which can contribute to pyrethrin decomposition.
Manufacturers extract and concentrate the active substances of pyrethrum from natural plant sources in extracts containing about 20% pyrethrins. Unfortunately, polyacetylenic substances are concentrated and removed from the plant residue along with the pyrethroid substances. There are two classes of naturally occurring polyacetylenes which contribute to the decomposition of pyrethrum. The two classes of polyacetylene substances are of the general formulae: EQU CH.sub.3 (C.tbd.C).sub.3 (CH.dbd.CH).sub.2 (CH.sub.2).sub.n CH.dbd.CH.sub.2 (I) EQU CH.sub.3 (C.tbd.C).sub.3 (CH.dbd.CH).sub.2 (CH.sub.2).sub.n O.sub.2 CCH.sub.3 (II)
wherein n=2 or 3. Hereinafter, when referring to one or the other class of polyacetylene substances the terms polyacetylene I or polyacetylene II will be used.